Recently, magneto-optical disks have been developed for use as optical memory elements that can record, reproduce, and erase information. The magneto-optical disk is constituted of a vertically magnetized film and a protective layer, formed on a substrate. Information recording on the magneto-optical disk is executed as follows: A light beam is projected onto the vertically magnetized film, causing a temperature rise at the illuminated area, and thus the magnetic coercive force of the area is lowered. In this state, an external magnetic field is applied onto the vertically magnetized film in such a manner that the direction of magnetization at the area where the coercive force lowered is inverted and aligned in the same direction as the external magnetic field, thereby permitting information to be recorded.
Recording methods in the magneto-optical disks are roughly classified into two methods: the light modulation method wherein recording is executed by modulating the intensity of a light beam in accordance with information to be recorded while simultaneously applying an external magnetic field in a constant direction; and the magnetic field modulation method, wherein recording is executed by inverting the direction of the external magnetic field in accordance with information to be recorded while applying a light beam of a constant intensity.
Here, the magnetic field modulation method is considered to be the most prospective as a method for achieving the so-called overwriting technique, wherein rewriting process allows new information to be written directly onto the previous information without the necessity of any erasing process. In adopting this overwriting technique, it is necessary to lower the inductance of the magnetic head with a view to increasing the switching speed of the external magnetic field direction and thereby enhancing the transfer rate. However, simultaneously, as the inductance of the magnetic head is lowered, the intensity of the magnetic field is also lowered and this necessitates that the magnetic head be disposed as close as possible to the magneto-optical disk.
When a resin such as polycarbonate is used as a substrate of the magneto-optical disk, considerable unevenness on the surface of the substrate can appear; that is, there exist protrusions and recessions in the circumferential direction. Because of this unevenness, if the magnetic head is located too close to the magneto-optical disk, the magnetic head might come into contact with the magneto-optical disk, causing damage to the magnetic head or the magneto-optical disk. Therefore, a certain amount of gap should be provided between the magnetic head and the magneto-optical disk, and this presents a problem in that it is difficult to obtain sufficient intensity of the magnetic field. Further, since the distance between the magnetic head and the vertically magnetized film is varied due to the unevenness, the intensity of the magnetic field tends to vary.
For this reason, as shown in FIGS. 48 and 49, a magneto-optical recording-reproduction apparatus, wherein a floating-type magnetic head 1 is adopted, has been proposed.
A floating-type magnetic head 1 is constituted by a magnetic head 8 (shown by hatching in FIG. 49) and a slider 7 which is provided with a magnetic head 8 and designed to glide above the magneto-optical disk 3.
The slider 7 is secured to the tip of a suspension 6 that is composed of plate springs, and is pressed toward the magneto-optical disk 3 by the suspension 6. The base of the suspension 6 is secured to a fixing member 5. The slider 7 is disposed so as to face the top surface of the magneto-optical disk 3, which is rotated by the spindle motor 2, and in this position, the magnetic head 8 is located to face an optical head 4, which is disposed below the bottom surface of the magneto-optical disk 3.
Seen from above, the slider 7 has a rectangular shape, and the magnetic head 8 is fixed to the rear end thereof. The size of the slider 7 is, for example, 5 mm long in the radial direction of the magneto-optical disk 3, and 7 mm long in the circumferential direction.
When the magneto-optical disk 3 rotates, an air flow is produced between the surface of the magneto-optical disk 3 and the floating-type magnetic head 1. Thus, the floating-type magnetic head 1 floats up to a height at which an upward floating force, caused by the air flow, and a downward pressing force, created by the suspension 6, balance with each other. With this arrangement, the gap between the floating-type magnetic head 1 and the magneto-optical disk 3 is kept virtually constant; therefore, the intensity of the magnetic field applied from the magnetic head 1 is not affected by the unevenness of the magneto-optical disk 3. Thus, stable recording operation can be performed on the magneto-optical disk 3.
However, in the case where the so-called CSS(Constant Start and Stop) method is adopted, wherein the floating-type magnetic head 1 comes into contact with the magneto-optical disk 3 upon floating or landing, problems are encountered in that the magneto-optical disk 3 and floating-type magnetic head 1 might be worn away or damaged due to repetitive CCS operations and, furthermore, the floating-type magnetic head 1 may stick to the magneto-optical disk 3, with the result that the spindle motor 2 is unable to rotate the magneto-optical disk 3.
These problems become more serious if the floating-type magnetic head 1 is used in a magneto-optical disk cartridge that is manufactured according to the ISO standard, a standard which is to be defined without anticipating the use of the floating-type magnetic head 1.
Furthermore, as shown in FIGS. 50 and 51, in the case where a ridge 3a exists near the outer edge of the magneto-optical disk 3, the movement of the floating-type magnetic head 1 is limited since it cannot pass the ridge 3a to the outer edge of the magneto-optical disk 3; thus, a problem is encountered whereby the storage capacity of the magneto-optical disk 3 is virtually reduced.
The ridge 3a occurs when a protective layer is formed through the spin-coat method. For example, a plurality of substrates made up of polycarbonate, each having 3.5 inches in diameter, are initially produced. Then, protective layers are formed on the respective substrates through the spin-coat method, and surface dimensions near the outer edge of each substrate are measured. The following FIGS. 52 through 55 show the results of the measurements.
In the magneto-optical disk 3 of FIG. 52, a ridge starts from a radial position of 41.5 mm from the center, and a maximum height of the ridge 3a is 16 .mu.m.
In the magneto-optical disk 3 of FIG. 53, a ridge starts from a radial position of 41.6 mm from the center, and a maximum height of the ridge 3a is 14 .mu.m.
In the magneto-optical disk 3 of FIG. 54, a ridge starts from a radial position of 41.6 mm from the center, and a maximum height of the ridge 3a is 12 .mu.m.
In the magneto-optical disk 3 of FIG. 55, a ridge starts from a radial position of 41.7 mm from the center, and a maximum height of the ridge 3a is 12 .mu.m.
Since the floating-type magnetic head 1 floats with a small gap from the surface of the magneto-optical disk 3, the floating-type magnetic head 1 comes into contact with the ridge 3a when it is used with the above magneto-optical disk 3 and moved to a radial position in the proximity of 41 mm (see FIGS. 50 and 51).
Additionally, in order to solve the above-mentioned problem of sticking, attempts such as providing a special texture treatment on the surface of the magneto-optical disk 3 or applying a lubricant thereto have been made.
Moreover, it has been reported in a digest C-27 of the Spring-time National Convention of the Institute of Electronics, Information and Communication Engineers of Japan,"Experiment of Landing on-off Flying Head Slider", and in a digest C-474 of the same convention (1990), "Flying Head Slider for Landing on-off", that the front and rear portions of the slider bottom surface were spherically machined in order to permit the floating head to land on/off without contacting against the disk.